1. Field of the Invention
The present invention relates to an ink jet head substrate which is effective for stable printing without causing malfunction against the noise, an ink jet head using said substrate, and an ink jet printing apparatus such as a printer using said head.
2. Related Background Art
An ink jet recording method (liquid jet recording method) is extremely superior in that the noise produced during operation is as little as to be ignorable, the high speed printing is enabled, and the so-called plain paper can be used for printing without need of a special treatment of fixing, and has become a main stream of the printing method.
In particular, a liquid jet recording method as described in, for example, Japanese Laid-Open Patent Application No. 54-51837 and Deutsche Offenlegungshrift No. 2843064 has a distinct feature in a respect that the motive force for discharging liquid droplets is obtained by applying thermal energy to the liquid, as opposed to other liquid jet recording methods, for example, a method of discharging liquid droplets by applying mechanical pressure.
That is, the recording method as disclosed in the above publications is characterized in that the liquid subjected to heat energy causes a state change with a rapid increase in volume to discharge liquid droplets through orifices at the top end of the ink jet head owing to action force based on said state change, and attach them to the recording medium to effect the recording.
Specifically, the liquid jet recording method as disclosed in Deutsche Offenlegungshrift No. 2843064 has the features that it is not only quite effectively applicable to a so-called drop-on-demand recording method, but also can provide the image with high resolution and quality at high rate because the ink jet head with a high density arrangement of discharge orifices and of the full-line type can be easily embodied.
The ink jet head applied to the above recording method comprises a liquid discharge portion having discharge orifices provided to discharge the liquid and liquid channels communicating to said discharge orifices, each having as its part a heat acting portion where heat energy for discharging liquid droplets is applied to the liquid, the liquid discharge portion being constituted of a head substrate (heater board) having electricity-heat converters (heating elements) as means for generating heat energy and a ceiling plate having grooves for forming discharge orifices and liquid channels.
In recent years, the head substrate has been constructed in a manner not only to have a plurality of heating elements on a substrate, but also provide, within the same substrate, respective heating element drivers, a shift register to transmit serially input image data to the respective drivers in parallel and having the same number of bits as the heating elements, and a latch circuit for temporarily storing data output from the shift register.
FIG. 4 shows an example of a conventional circuit configuration on the substrate. Herein, 400 is a substrate, 401 is a heating element, 402 is a power transistor, 403 is a latch circuit, and 404 is a shift register. In addition, for the purpose of the miniaturization of a printer main power source by reducing the number of heating elements to be driven simultaneously to decrease instantaneous current flow, there is provided a time-division driving block selecting logic 405 such as a decoder provided to divide a group of heating elements into blocks each consisting of a predetermined number of elements and make the division driving of each block as a unit, and a logic system buffer 406. The input signals include those for the clock of operating the shift register, the image data input of receiving image data in serial, the latch clock of holding data in the latch circuit, the block enable of block selection, the drive pulse (heat pulse) width input of controlling externally the ON time of the power transistor, i.e., the time for driving the heating elements, a logic circuit drive power source (5V), GND, and a heating element drive power source, these signals being input via pads 407, 408, 409, 410, 411, 412, 413 and 414 on the substrate, respectively.
A drive sequence includes first transmitting image data from the printer main device in synchronism with the clock and serially to the substrate within the head, which data is read by the shift register 404 within the substrate. The read data is temporarily stored in the latch circuit 403 to make the block selection in time division until next image data is held in the latch circuit. At each block selection, if a pulse is input from the heat pulse 411, the block selection is performed, and if image data is on, one or more power transistors 402 are turned on; and said block selection is made, and if image data is on, current is flowed through one or more heating elements to effect the driving.
As above described, the integration of the logic circuit such as a driver, a shift register, a latch, etc. into the head substrate has recently progressed, but the current pulse flowing through each heating element reaches 100 to 200 mA instantaneously, and for example, if the heating elements turning on at the same time are eight elements, a current pulse of about 1 to 1.5 A will flow through the heating element drive power source line and the GND line. The problem herein encountered is that the logic circuit on the head substrate may cause malfunction due to the noise with inductive coupling produced in the flexible wiring from the printer main device to the ink jet head or the wiring within the ink jet head.
Herein, though the noise with capacitive coupling is naturally apprehended, the clock frequency of the ink jet head is roughly at most several MHz, and if the logic power source voltage is about 5V, there is only a small possibility of having effect on the operation, in which the former inductive noise will have more effect to cause the malfunction. In particular, when the clock or the latch clock within the head substrate malfunctions due to the noise, there is a high possibility that the image data within the head substrate is completely different from the data transmitted from the printer main device, significantly having detrimental effect on the print quality. Since the level of inductive noise is higher with larger variation of current per unit time, if the number of discharge orifices is increased for the higher speed printing, it is expected that the number of elements turned on simultaneously is further increased, so that the current value of the current pulse is further increased and the noise level is raised.
To resolve such a problem, some measures are conceived. One example is to reduce the number of heating elements turned on simultaneously by increasing the number of blocks to restrain the magnitude of the current pulse. However, in making the high speed printing, the interval of holding data by the latch circuit from one time to the next, that is, the discharge period, is shortened, so that the time allocated to each block is shortened by the increased number of blocks, and there is a risk that sufficient energy to discharge the ink may not be obtained.
Another resolution is also conceived which involves providing a capacitor for the current supply on or around a carriage itself for the printer main device supporting the ink jet head to reduce the inductive noise on the flexible substrate, or adding a noise countermeasure component to prevent malfunction, and in practice, there are many cases of adopting such a measure in the carriage portion for the ink jet printer. In such a case, however, the larger size of the carriage portion with this measure can not be avoided, resulting in a problem that the printer main device can not be reduced in size and the cost for the countermeasure component may be increased.
The above problem may be observed not only in an ink jet head with the heating elements arranged at high density and capable of attaining the high speed printing, but also other print heads, for example, a thermal head having heating elements arranged lengthwise or a print head having recording elements driven by the driving pulse arranged, which may cause malfunction due to the noise.